In image forming apparatuses which use an electrophotographic method, such as, copying machines, printers, and MFPs (Multi-Function Peripheral), a fixing apparatus is used for fixing transferred toner onto a sheet.
For example, the fixing apparatus heats and pressurizes the sheet passing through a nip formed between two rollers thereby fixing the toner onto the sheet. Of the two rollers, the roller that mainly heats the sheet is called a fixing roller (or a heating roller), and the roller that mainly pressurizes the sheet is called a pressurizing roller. In addition, there are many configurations in which a belt called a fixing belt is provided separately from the fixing roller so that the fixing belt is interposed at the nip between the fixing roller and the pressurizing roller. In this configuration, the sheet to be fixed passes through a nip between the fixing belt and the pressurizing roller.
In the configuration having the fixing belt, not the fixing roller but the fixing belt is directly heated by a heater. As heat of the heated fixing belt is transferred to the sheet, the toner is fixed onto the sheet. During the fixing, since the heat of the fixing belt is lost to the sheet, the temperature of a portion of the fixing belt which comes in contact with the sheet is temporarily reduced.
Sizes of the sheets to be fixed are not always definite. Various sizes of sheets may be mixed. For example, there may be a case where immediately after a sheet with A4 size is fixed, a sheet with A3 size is fixed. In this case, immediately after the fixing of the A4 sheet, in the rotation shaft direction of the fixing roller, that is, in the width direction of the fixing belt, even though the temperature of a region corresponding to the A4 size width is reduced, the temperatures of both the side regions are maintained at a high temperature. That is, non-uniformity of temperature occurs in the width direction of the fixing belt. Therefore, if fixing of the A3 size sheet is subsequently performed in this state, the center portion is fixed at a low temperature, and conversely both end portions are fixed at a high temperature, such that the fixing state becomes non-uniform in the width direction of the sheet (in a direction perpendicular to a carriage direction).
Therefore, in order to solve the problem, a fixing apparatus which can always maintain uniformity of heat in the width direction of a fixing belt even when the temperature of the fixing belt in the width direction is partially reduced, so as to immediately return the temperature to the entirely uniform temperature is desirable.